1. Field of the Invention
This invention relates to a liquid crystal composition, particularly to a liquid crystal composition suitable for use in twisted nematic (TN) liquid crystal display devices of the multiplex drive type, and to a liquid crystal display device using the liquid crystal composition mentioned above.
2. Description of the Prior Art
Recently, the liquid crystal display devices have been desired to possess larger capacities for the information to be displayed. The demand for liquid crystal display devices, therefore, has been gradually shifting from those of the segment display type to those of the matrix display type. For the matrix type display device to enjoy addition to its capacity for the information to be displayed, however, it is inevitably required to increase the capacity of the so-called multiplex drive. Unfortunately, such addition to the capacity entails decline of display contrast, decrease in the range of visual angle, and degradation of response characteristics.
First, as a measure to preclude the decline of display contrast, display characteristics of the liquid crystal display devices above the threshold voltage should be sharply changed. To provide an increase in the sharpness, the liquid crystal composition is required to possess as low elastic constant ratio, k.sub.33 /k.sub.11 (wherein k.sub.11 stands for splay and k.sub.33 for bend), as possible and as low dielectric anisotropy, .DELTA..epsilon.(.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. ; wherein .epsilon..sub..perp. stands for the dielectric constant in the direction of long axis of molecule and .epsilon..sub..perp. for the dielectric constant in the direction of short axis of molecule) as possible.
Then, as a measure to curb the decrease in the range of visual angle, it is necessary that the liquid crystal composition should possess as low birefringence, .DELTA.n, as possible.
If the birefringence, .DELTA.n, is lowered more than is necessary, however, the incident beam of light is not bent and is caused to pass through the liquid crystal display device, with the results of the coloration and the poor contrast. So, the display quality of the liquid crystal display device is seriously impaired. The birefringence, .DELTA.n, of the liquid crystal composition, therefore, should be fixed with due consideration to the adaptability of the anisotropy to the optical conditions of the display device.
The response and recovery times of the liquid crystal display devices are directly proportional to the magnitude of the viscosity, .eta., of the material for liquid crystal. To enhance the response characteristic of the liquid crystal display device, therefore, it is necessary that the liquid crystal composition should possess as low a viscosity as possible.
It is desirable to produce a liquid crystal composition which satisfies the requirements mentioned above and, at the same time, exhibits an ample range of liquid crystal temperature.
The liquid crystal compositions which are available in the market and which are known in the art to be suitable for use in liquid crystal display devices of the multiplex drive type are represented by those liquid crystals produced by Hoffmann-LaRoche & Co., Ltd. (such as RO-TN-200, -201, -211, -103, -132, -403, and -430) [cf. IEEE Trans. Elec. Devices, ED-25 (1978) 1125] and those liquid crystals made by BDH Chemicals Ltd. (such as E-90, -92, -94, -96, -98, -100, -102, -104, -106, -108, and -110) [cf. A leaflet of liquid crystal materials by BDH Chemicals Ltd. (UK), Feb. of 1981]. None of these known, commercially available liquid crystal compositions, however, fully satisfies all the requirements mentioned above.
As the results of various and extensive studies, we have discovered the liquid crystal composition usable for the matrix drive display device, by selecting and utilizing liquid crystal compounds possessing the following five properties, namely, low viscosity, high melting point, positive dielectric anisotropy, low birefringence, and low elastic constant ratio.